The present invention relates to an optical amplifier relay transmission system, especially to an optical amplifier relay transmission system using erbium-doped fiber amplifiers (EDFAs) capable of improving transmission of a monitoring/control signal used for the monitoring control of an optical system.
In the prior art transmission system of this type, the optical signal to be transmitted is amplified by the EDFAs in relay stations. A monitoring/control signal for monitoring and control of the optical signal is multiplexed to a main signal in an overhead of the optical signal and transmitted. For the monitoring and control of the optical signal, each relay station has to extract the monitoring/control signal that is inserted in the overhead of the optical signal. To this end, each relay station once converts the optical signal including the main signal into an electric signal, then separates the monitoring/control signal from the electric signal for predetermined processing of the separated monitoring/control signal. The relay station then multiplexes again the processed monitoring control signal to the main signal, then converts the multiplexed signal to an optical signal again, and amplifies the optical signal before transmission. Briefly, each relay station converts the main signal as well to an electric signal and converts the electric signal back to the optical signal. Therefore, a large scale optical signal processing circuit is required.
To overcome this drawback, a system has been proposed, in which an optical signal obtained through wavelength multiplexing using a different frequency from that of the main signal for the monitoring/control signal. In this system, the monitoring/control signal can be separated from the main signal by wavelength separating the optical signal, and the separated monitoring control signal may be subjected to a predetermined processing and wavelength multiplexed again afterwards. This is effective in view of reducing the processing circuit scale in each relay station.
In the prior art transmission systems of this type, a single transmission line constituted by an optical fiber is utilized for the optical signal transmission. Therefore, it is difficult to promptly cope with the occurrence of such troubles as breakage of the optical fiber or a transmission line error. To cope with this difficulty, a system is adopted, in which upon occurrence of a trouble in an optical transmission line, switching thereof over to another optical transmission line is executed. In this system, for realizing reliable transmission it is necessary, upon detection of a trouble, to re-transmit a signal which has been affected by the trouble. Therefore, the transmission efficiency of the system is inferior. Besides, in case when the monitoring/control signal fails to be transmitted, the system can no longer make monitoring or control, and the switching over to the redundant circuit noted above is inadequate to disable effective protection of the system against the trouble.